1. Field of the Invention
This invention relates to a filled, hydrocarbon polymer composition containing a synergistic mixture of at least two interfacial agents which have a reinforcement promoting effect on the filled polymer. Reinforcement promotion refers to the simultaneous increase in tensile strength and ductility properties of a filled polymer composition.
2. Description of the Prior Art
A broad range of chemicals have been evaluated as filler treatments or interfacial agents in filled polymers with and without the addition of free radical initiators, such as peroxides. Unfortunately, the literature terminology is usually ambiguous and often erroneous. For example, the terms "coupling agent" or "adhesion promoter", which imply that the additives increase the adhesion or bonding between the filler particle and the surrounding polymer matrix, are often used uncritically. Usually there is no proof of any adhesion effect, and the particular additive may function merely as a filler dispersing aid and, sometimes, as a processing aid by reducing the viscosity of the molten, filled composite. In many cases, the mechanical properties reported for the filled composites even imply that the additive facilitates release of the matrix polymer from the filler particles, such that the so-called coupling agent actually has a decoupling or debonding effect.
The varied behavior of filler treatment additives in filled polymers may be more clearly envisioned with the help of a composite property chart such as that shown in the FIGURE. On this chart, the abscissa or "x" axis represents the elongation at break and the ordinate or "y" axis represents the maximum tensile strength of a filled polymer.
The interfacial agents which fall in the area around "A" in the FIGURE are ineffective and the mechanical properties of a filled polymer composition are essentially unchanged from those of a controlled composition without any interfacial agent. In general, these compositions have little, if any, practical interest in and of themselves.
Certain currently used interfacial agents and filler treatments result in increases in tensile strength with little or no increase in elongation at break, such that although the materials get stronger, they remain brittle. These compositions are grouped in the area from "A" to "B" and the corresponding interfacial agents will here be called coupling agents in the strict sense of the word. This area is rather sparsely populated, usually by coupling agents which have been developed specifically to provide adhesion for a particular polymer and substrate combination. Increased interfacial adhesion, i.e., coupling, is believed to be responsible for the increased tensile strength, but the polymer matrix still has to accommodate the strain of the total composition, such that the ductility is usually improved only slightly, and sometimes even reduced. The compositions in the area from "A" to "B" in the Figure can, therefore, occasionally be of practical value, especially in cases where the virgin, i.e., unfilled, polymer has high ductility so that the filled polymer still obtains a useful combination of mechanical properties.
Other commonly used additives result in gains in elongation break with little changes, or even decreases, in tensile strength, such that although such compositions can be more ductile, they remain weak, and are often best characterized as "cheesy". These compositions are grouped in the area from "A" to "C", and the corresponding interfacial agents will here be termed decoupling agents. These compositions are fairly well represented by many so-called lubricants, processing aids, detergents, waxes and the like. The low interfacial adhesion is believed to be the principal reason for these characteristics in contrast to the composites in the area from "A" to "B". In this case, the load-bearing capability is impaired because the filler particles contribute little, if anything, to load capacity, since they separate from the matrix at the application of very modest loads. On the other hand, this microcavitation effect, which often is visible as stress-whitening, reduces the actual matrix strain, such that the compositions can have substantially improved ductility. The compositions in this area may be useful for non-load bearing applications, such as where a plastic part is overdesigned from a stress standpoint. The improved ductility can be very useful for parts which must be able to withstand high impact loads without fracturing, but would be useless for structural applications, especially those requiring resistance to creep during long term static loading.
Clearly, the interfacial agents which cause a filled polymer to become both strong and tough, i.e., which cause improvements in both tensile strength and elongation at break, are by far the commercially most attractive compounds. These compositions would be grouped in the area from "A" to "D", which is the least populated area in the FIGURE. The interfacial agents in this area are the ones which have been termed "reinforcement promoters". These materials are commercially very valuable because they can be used broadly to formulate filled thermoplastic compositions for applications requiring both load-bearing capability with resistance to creep, and at the same time requiring toughness, i.e., resistance to fracture by impact load. Furthermore, filled polymers in this area are often much less expensive in terms of raw material costs than are unfilled, engineered polymers, which they would be capable of replacing.
Not all interfacial agents can be clearly defined as reinforcing, coupling, decoupling, or ineffective since, as can be seen from the FIGURE, no sharp boundaries exist between the designated areas. This is particularly evident for agents providing only modest increases in strength or elongation at break, i.e., which approach the area around "A" in the FIGURE, or where the reinforcement promotion is attempted under low sensitivity conditions such as for compositions having relatively low filler loadings. This ambiguity, however, does not detract from the facts that there are highly effective agents in each of the three areas, out to "B", "C" and "D", in the FIGURE; that these agents differ greatly in how they affect the properties of a filled polymer; and that these differences are of considerable commercial importance. In addition, while some effects are commonplace and not of much practical interest, other effects are quite difficult to achieve and at the same time highly desirable. An improvement in the area of reinforcement promotion for filled polymers is presented in copending patent application Ser. No. 295,811, entitled "Reinforcement Promoters for Filled Thermoplastic Polymers", now U.S. Pat. No. 4,385,136 (Ancker et al.), issued May 24, 1983, filed with this application and is incorporated by reference herein. The present invention is a further improvement in reinforcement promotion of filled polymers.